Phase 5: Add conservative path filters to skip unaffected CI work

[dgee2]

Summary

Use path-based conditions so frontend and backend jobs only run when relevant files change.

Why

This improves average CI throughput across the repo, even if it does not materially change every single run.

Guardrails

Keep filters conservative to avoid missing cross-stack changes.

Acceptance criteria

• Frontend runs for ui/**, OpenAPI contract changes, and workflow changes
• Backend runs for .cs, .csproj, solution, and workflow changes
• No required validation is skipped for cross-stack changes